Engines may be configured with exhaust gas recirculation (EGR) systems to divert at least some exhaust gas from an engine exhaust manifold to an engine intake manifold. By providing a desired engine dilution, such systems reduce engine knock, throttling losses, in-cylinder heat losses, as well as NOx emissions. As a result, fuel economy is improved, especially at higher levels of engine boost. Engines have also been configured with a sole cylinder (or cylinder group) that is dedicated for providing external EGR to the engine cylinders. Therein, all of the exhaust from the dedicated cylinder group is recirculated to the intake manifold. As such, this allows a substantially fixed amount of EGR to be provided to engine cylinders at most operating conditions. By adjusting the fueling of the dedicated EGR cylinder group (e.g., to run rich), the EGR composition can be varied to include species such as hydrogen which improve the EGR tolerance of the engine and result in fuel economy benefits.
Various approaches may be used to time the firing of one or more dedicated EGR cylinder with non-dedicated EGR cylinders. In one example, the firing of the non-dedicated EGR cylinders may be distributed unevenly due to the firing of the dedicated EGR cylinder(s). For example, US patent publication no. 2012/0216530 shows firing a dedicated EGR cylinder between successive firing of three non-dedicated EGR cylinders. Additionally, all cylinders, including the dedicated EGR cylinders, fire at the same number of crank degrees after the previously fired cylinder. However, the inventors herein have recognized problems with this approach. For example, firing non-dedicated EGR cylinders and dedicated EGR cylinders with a conventional cylinder firing interval results in uneven exhaust pulses being delivered to the turbine. Further, a period may occur, during the firing of the dedicated EGR cylinder, where no exhaust flows to the turbine. Uneven exhaust blowdown pulses at the turbine may result in reduced engine performance and/or compressor surge events. Further still, in some examples, exhaust valve overlap between two or more cylinders routing exhaust to the turbine may result in exhaust blowdown interference at the turbine, thereby increasing in-cylinder pressures and decreasing engine efficiency.
In one example, the issues described above may be addressed by a method for operating a first set of exhaust valves of a first group of cylinders to have no exhaust valve opening overlap and operating one or more second exhaust valves of a dedicated EGR cylinder routing exhaust to an intake manifold so that opening of the one or more second exhaust valves overlaps with opening of exhaust valves of two cylinders of the first group of cylinders. In this way, exhaust blowdown interference between the first group of cylinders may be decreased and exhaust pulses to the turbine may be more evenly spaced.
For example, the first group of cylinders may be a group of non-dedicated EGR cylinders routing exhaust via the first set of exhaust valves to a turbine while the dedicated EGR cylinder is the only cylinder routing exhaust to the intake manifold. By operating the non-dedicated EGR cylinders to have no exhaust valve overlap via changing the firing interval of those cylinders, exhaust valves of different cylinders for the first group of cylinders may not be open at the same time. Further, by overlapping opening of the exhaust valve of the dedicated EGR cylinder with two exhaust valve opening events of two of the non-dedicated EGR cylinders, exhaust may continue to flow to the turbine even while the exhaust valve of the dedicated EGR cylinder is open. In this way, the exhaust pulses to the turbine may be spaced more evenly without periods of no exhaust flow to the turbine. Additionally, exhaust exiting the exhaust valves of the non-dedicated EGR cylinders may not interfere with exhaust exiting the other exhaust valves of the non-dedicated EGR cylinders, thereby increasing engine efficiency.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.